1. Field of the Invention
The present invention is in the field of power supplies, and more specifically relates to a power supply suitable for driving a resistive load that varies widely.
2. The Prior Art
A simple, but wasteful, way of achieving the desired result is to place in series with a resistive load a resistor having a resistance that is much greater than the maximum load resistance. Thereby, variations in the load resistance cause only a small percentage change in the total resistance of the circuit and hence in the current that is flowing. The relatively large amount of power dissipated in the added resistor renders this technique undesirable in many applications.
An extension of this technique is often used in power supplies for fluorescent lamps wherein an inductor, also called a ballast, is placed in series with the lamp. In that application, the inductor is useful in permitting the full applied voltage to be impressed on the source initially, to strike an arc within the lamp, and is useful thereafter in limiting the current flow through the lamp after the arc has been struck. The inductor is not as wasteful as the resistor referred to earlier, because the inductor returns the energy stored in it at some point in the cycle.
However, the inductor has the disadvantages of being heavy, because of its windings and possible iron core, and in practice, the inductors must be rather large and heavy, and therefore difficult to mount.
A specialized apparatus known as a metadyne is sometimes used in electric traction systems. It involves the use of a single continuous rotating winding, a commutator, and two sets of brushes. Although the metadyne provides constant torque for traction, it does this only at the expense of requiring massive rotating machinery.
As will be seen below, the present invention overcomes the difficulties inherent in each of these prior art approaches, achieving high efficiency, without the need for any moving parts.